This invention relates to a mechanism for loading a film back in or unloading it from a camera body.
When a conventional film back is loaded into a camera body, a gear operated in association with the winding mechanism in the camera body is engaged with the spool gear which is fixedly mounted on the spool shaft of the film back, so that the winding force can be transmitted from the camera body to the film back to wind the film on the spool. In many cases, the spool gear is so designed that it is allowed to turn in one direction only, in order to prevent the film wound on the spool from being slackened.
FIGS. 1 and 2 show one example of a conventional mechanism adapted to cause the aforementioned spool gear to turn in one direction only. The spool gear 1 is provided on one side of a film back 2 and is fixedly secured to one end of a spool shaft 3 which is rotatably supported by both sides of the film back 2. When the film back 2 is loaded in a camera body 4 (by being moved in the direction of the arrow A), the spool gear 1 is engaged with a gear 5 which is operated in association with the winding drive mechanism of the camera.
A ratchet member 6 permitting the spool gear 1 to turn in one direction only is rotatably supported by the side of the film back 2. The ratchet member 6 has a pawl 6a and an elongated slot 6b. The pawl 6a is engaged with a tooth of the spool gear 1. One end of a spring 7 which is wound on a small-diameter portion 1b of the spool gear 1 is inserted into the elongated slot 6b so as to bias the pawl in the counterclockwise direction but permit the ratchet member 6 to intermittently operate. A stopper 8 is provided near the pawl 6a on the side of the film back 2 in order to limit the intermittent motion of the ratchet member 6. Therefore, the spool gear 1 shown in FIGS. 1 and 2 is allowed to turn counterclockwise only, and it is not allowed to turn clockwise by the pawl 6a.
However, the one-way-turn system of the spool gear described above suffers from several difficulties. Since the spool gear 1 is permitted to turn in one direction only as described above, when the film back 2 is loaded in the camera to allow the spool gear 1 to engage with the gear 5 on the camera, the spool gear 1 cannot be smoothly engaged with the gear 5 if a force acts on the spool gear 1 in the direction in which the rotation thereof is inhibited. Further, if the film back 2 is unloaded from the camera body 4 (by being moved in the direction of the arrow B), the spool gear 1 cannot be smoothly disengaged from the gear 5. Thus, it is difficult to smoothly load or unload the film back. Furthermore, for the same reasons, the feeling which the operator gets in winding the film may be unsatisfactory.